1. Field of the Invention
The present invention relates to a water quality evaluation method of ultrapure water applicable in the industry of electronic materials such as semiconductor and liquid crystal. More particularly, the present invention relates to a preferred evaluation method in the condition that the water quality of ultrapure water is evaluated by contacting the ultrapure water and a substrate, such as a semiconductor wafer, to perform surface analysis on the substrate. The present invention further relates to a substrate contacting apparatus used by the water quality evaluation method.
2. Description of Related Art
Ultrapure water used in the field of electronic industry is the last substance contacting wafers in a cleaning step, so the concentrations of impurities contained in the ultrapure water will have influence on the cleanliness of the surfaces of substrates represented by silicon. Therefore, in the semiconductor field, with the increase of integration, the concentration of impurities in the ultrapure water used during the manufacturing must be reduced. In recent years, researchers have endeavored to reduce all the impurities contained in the ultrapure water. Accordingly, a technique of analyzing ultra-trace impurities in the water by using an analysis device with high sensitivity has been developed.
However, in recent years, with the sharp improvement of the performance of semiconductor products, even ultrapure water, in which ultra-trace impurities have been definitely analyzed, is used to perform cleaning step, the quality standard of the product still cannot be satisfied.
The organics that are likely to affect the quality of the semiconductor products are various and cannot be determined, and the amount of the impurities in the ultrapure water is of ng/L scale and being ultra micro and is lower than or equal to the lower limit of analysis, so it is difficult to control the water quality. Additionally, even in a circumstance that a substance exists in the ultrapure water in an extreme micro amount, if the material is easily to be attached on the substrate due to co-existing substances, it will have undesired influence on the substrate, which is difficult to be controlled even the ultrapure water quality is analyzed directly, so the water quality evaluation cannot be properly performed.
In order to solve this problem, a water quality evaluation method is developed in the industry, which includes contacting a substrate such as a semiconductor wafer with water to be evaluated such that the impurities in the water to be evaluated attach on the substrate; analyzing the attachments on the substrate surface, or eluting the attachments and analyzing the eluant, or analyzing the changes on the substrate surface, so as to evaluate the water quality.
For example, a method of detecting metals attached on a substrate surface by using total reflection fluorescence X-ray analysis is disclosed in JP Patent Publication No. 2001-208748. Additionally, a method of detecting organics attached on a substrate surface by using Fourier transform infrared spectrometry (FTIR) or thermal desorption gas chromatograph mass spectrometer (TDGCMS) is disclosed in JP Patent Publication No. 2005-274400.
According to the above water quality evaluation methods, the influence of the water to be evaluated on the substrate is actually known, and thus the impurities in the water to be evaluated which have influence on the substrate can be determined.
Moreover, according to JP Patent Publications No. 2001-208748 and No. 2005-274400, the substrate is accommodated in a retention container (substrate contacting apparatus), and water to be evaluated is fed into the retention container to contact the substrate, and then the substrate is taken out of the retention container and placed in a sealed container and transported to an analysis device to be analyzed.    [Reference 1] JP Patent Publication No. 2001-208748.    [Reference 2] JP Patent Publication No. 2005-274400.
According to the evaluation method of JP Patent Publication No. 2001-208748 and No. 2005-274400, in order to prevent impurities in the air from attaching on the substrate surface when taking the substrate out of the retention container, a step of feeding water to the retention container must be performed in a place having clean gas environment, such as a clean room, or near such a place.
Preferably, except to opening the retention container to accommodate the substrate in the container or take the substrate out of the container in a clean room, in any steps, the retention container accommodating the substrate must be completely sealed.
That is to say, after accommodating the substrate in the retention container in a clean room, or when transporting the retention container to a position where the water to be evaluated contacts with the substrate (the water to be evaluated will be fed into the retention container), or when feeding the water to be evaluated into the retention container, and when transporting the retention container with the substrate accommodated therein to an analysis device after stopping feeding water, it is desired to keep the substrate in the retention container isolated from the air (external gas).
Particularly, when equipment with high precision is required to perform the substrate surface analysis and the place of the analysis device is limited, it takes some time to transport the retention container to the analysis device after contacting the water to be evaluated and the substrate, and the available time of the analysis device is limited, so the substrate must be kept in the retention container for a long time. Therefore, it is desired to keep the retention container to be sealed for a long time.
Furthermore, if the interior of the retention container cannot be keep at an enough vacuum degree, a little amount of air still contacts the substrate in the retention container, thus causing changes on the substrate surface state due to the contaminations and oxygen gas in the air. As a result, it is difficult to determine whether the changes on the substrate surface state are caused by the water to be evaluated or the contact with the air. Moreover, if the substrate surface state changes due to the contact with the water to be evaluated and further changes due to the contact with the air, it is difficult to evaluate water quality with high precision (to determine the amount of metals or organics in the water to be evaluated).
For example, if alkali substances, such as ammonia and amine, exist in the water to be evaluated, depressions and protrusions will be formed on the substrate surface due to etching effect. However, when the air contacts the substrate surface, the contaminations in the air are attached on the substrate surface, or an oxide film is formed on the substrate surface due to the oxygen gas in the air, the height of the depressions and protrusions is reduced.